| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 97 === We report a fabrication method to realize transparent thin film transistors (TFTs) at room temperature using radio frequency (RF) sputtered In2O3/Ga2O3 materials. The transmission line method (TLM) was used to examine the relationship between the resistivity of oxide channels and the growth conditions. We note that the oxide TFTs with channel resistivity lying between 4.4×10^4 Ω-cm to 6.56×10^5 Ω-cm exhibit better transistor characteristics. The gate insulator was formed by a thin silicon nitride layer grown by plasma enhanced chemical vapor deposition (PECVD) at low temperature. Considering the work function difference between the metal and the indium oxide, We selected molybdenum (Mo) as the source, drain, and gate contact metals. For an In2O3 TFT with a channel length and channel width of 8 um and 80 um, the device is operated at a depletion mode with a threshold voltage of -2.2 V. The electrical characteristics also exhibit the following: an on-to-off current modulation ratio (Ion/Ioff) of ~1.6×10^6, a subthreshold swing (S) of 1.3 V/decade, and a field-effect mobility (uFE) of 1.98 cm^2V^-1s^-1.
We further proposed a new device design by including a supper-lattice channel structure to improve the transistor characteristics at room temperature. Using incorporating an In2O3/Ga2O3 super-lattice channel structure, the transistor is shown to operate in an enhancement mode with a threshold voltage of 4.5 V. The device exhibits improved characteristics: with the on-to-off current modulation ratio (Ion/Ioff) increased from ~10^6 to 10^7, and the saturation current 30 uA (W/L=80 um/8 um). The subthreshold swing (S) and field-effect mobility (uFE) are estimated as 0.66 V/decade and 0.3~1.02 cm^2V^-1s^-1, respectively. All The TFT channels are transparent in visible wavelength (350 nm~750 nm) with transmittance over 80%.
We further provide CV analysis to show that the interface trap density is 10^12~10^13 cm^-2eV^-1 for our TFTs, and the mobile charge density is 8.85×1011 cm-2. The reasons can be ascribed to the low temperature grown of silicon nitride. The high level mobile charge may be one of the reasons to limit the mobility of our TFT devices. However, the super-lattice TFTs offer good transistor characteristics to serve as candidate for active switch for flexible electronic applications.
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